On December 28, 1778, Herbert was concerned in a great attempt to escape. A hole nine feet deep was dug by the side of the inner wall of the prison, thence for fifteen feet until it came out in a garden on the other side of the road which bounded the outer wall. The difficulty of getting rid of the excavated dirt was great, and, moreover, excavation could only be proceeded with when the guard duty was performed by the Militia regiment, which was on every alternate day, the sentries of the 13th Regular regiment being far too wideawake and up to escape-tricks. Half the American prisoners—some two hundred in number—had decided to go. All was arranged methodically and without favour, by drawing lots, the operation being conducted by two chief men who did not intend to go. Herbert went with the first batch. There were four walls, each eight feet high, to be scaled. With five companions Herbert managed these, and got out, their aim being to make for Teignmouth, whence they would take boat for France. Somehow, as they avoided high roads, and struck across fields, they lost their bearings, and after covering, he thinks, at least twenty miles, sat down chilled and exhausted, under a haystack until daybreak. They then restarted, and coming on to a high road, learned from a milestone that, after all, they were only three miles from Plymouth! Day came, and with it the stirring of the country people. To avoid observation, the fugitives quitted the road, and crept away to the shelter of a hedge, to wait, hungry, wet, and exhausted, during nine hours, for darkness. The end soon came. In rising, Herbert snapped a bone in his leg. As it was being set by a comrade, a party of rustics with a soldier came up, the former armed with clubs and flails. The prisoners were taken to a village, where they had brandy and a halfpenny cake each, and taken back to Plymouth. At the prison they learned that 109 men had got out, of whom thirty had been recaptured. All had gone well until a boy, having stuck on one of the walls, had called for help, and so had given the alarm. Altogether only twenty-two men escaped. Great misery now existed in the prison, partly because the charitable fund had been exhausted which had hitherto so much alleviated their lot, and partly on account of the number of men put on half allowance as a result of their late escape failure, and so scanty was food that a dog belonging to one of the garrison officers was killed and eaten. Herbert speaks in glowing terms of the efforts of two American ‘Fathers’, Heath and Sorry, who were allowed to visit the prison, to soften the lot of the captives. Finally, on March 15, 1779, Herbert was exchanged after two years and four months’ captivity. In a table at the end of his account, he states that between June 1777, and March 1779, there were 734 Americans in Mill Prison, of whom thirty-six died, 102 escaped, and 114 joined the British service. Of these last, however, the majority were British subjects.